1. Field of the Invention
The present invention relates to a phase shift circuit, and more specifically to a phase shift circuit suitable for use in an FM detecting circuit of quadrature detection method adopted for a portable FM radio receiver, for instance.
2. Description of the Prior Art
FIG. 1 shows a conventional FM detecting circuit of quadrature detection method. In the drawing, an FM broadcasting radio wave coming to an antenna is amplified by a high frequency amplifier circuit (not shown), and then the frequency of the amplified radio wave is converted into an intermediate frequency signal by a frequency converting circuit (not shown). The obtained intermediate frequency signal is given to an intermediate frequency amplifier 1 of a differential type, and the output of the intermediate frequency amplifier 1 is separated into two intermediate frequency signals having 90-degrees out of phase with respect to each other through a phase shift circuit 2. The two obtained signals are applied to a multiplier 3 for outputting a multiplication value according to the frequency deviation from the intermediate frequency. This output is further smoothed by a low-pass filter (not shown), and further demodulated to a level signal according to the frequency thereof.
With reference to FIG. 1, the phase shift circuit 2 is described in more detail hereinbelow. The differential outputs of the intermediate frequency amplifier 1, that is, the positive-phase output signal and the negative-phase output signal are given to the bases of two transistors Q1 and Q2 via coupling capacitors C1 and C2, respectively. The respective bases of the transistors Q1 and Q2 are connected to a dc supply voltage Vcc via resistors R1 and R2, respectively for application of a bias voltage thereto. The respective collectors of the transistors Q1 and Q2 are connected to the voltage supply Vcc. Further, the emitters of the two transistors Q1 and Q2 are connected to current sources I1 and I2, respectively. Between the two emitters of the transistors Q1 and Q2, a series-connected resistors R3 and R4 are connected. The resistance ratio of the two resistors R3 and R4 is set to 3:1, as described later. A voltage V1 obtained at the junction point between the two resistors R3 and R4 is applied to the base of a transistor Q3 for constituting a differential amplifier together with a transistor Q4. The emitter of the transistor Q2 is connected to the base of the transistor Q4 via a resistor R7. The respective collectors of the transistors Q3 and Q4 are connected to the supply voltage Vcc via resistors R5 and R6, respectively. The base of the transistor Q4 is connected to the supply voltage Vcc via a ceramic resonator Zc. The ceramic resonator Zc is so determined that the frequency at roughly an intermediate point between the resonant point and the anti-resonant point thereof becomes the intermediate frequency and further the absolute value of the impedance at the intermediate frequency becomes equal to the resistance of the resistor R7. The respective emitters of the transistors Q3 and Q4 are connected to a current source I3. The respective emitter outputs of the emitter-follower transistors Q1 and Q2 are inputted to the multiplier 3 as a first pair of the input signals, and the respective outputs of the transistors Q3 and Q4 (e.g., the phase shift circuit 2) are inputted to the same multiplier 3 as a second pair of the input signals.
In the FM detecting circuit of quadrature detection method as described above, in order to obtain signals whose phases are shifted 90 degrees out of phase with respect to each other, the ceramic resonator Zc designed so as to be vibrated at the intermediate frequency (e.g., 10.7 MHz) is used to eliminate the phase adjustment.
In the above-mentioned circuit configuration, the reason why two 90-degree out-of-phase signals can be obtained will be explained hereinbelow. The differential output signals of the intermediate frequency amplifier 1 are inputted to the bases of the transistors Q1 and Q2 which operate as an emitter follower circuit. Therefore, two opposite-phase signals can be developed at the emitters of the two transistors Q1 and Q2. Here, when the resistance ratio of the resistors R3 and R4 is determined to be 3:1, the relationship between the signal V1 applied to the base of the transistor Q3 and the signal V2 outputted from the emitter of the transistor Q2 can be expressed as EQU V1=(1/2).multidot.V2
Further, for instance, if the use frequency of the ceramic resonator is 10.7 MHz and the inductive impedance .vertline.Zc.vertline. thereof is 1 k.OMEGA. and further if the resistance of the resistor R7 is 1 k.OMEGA., the signal V3 developed at the base of the transistor Q4 can be expressed as ##EQU1##
Further, the output voltage of the differential amplifier composed of the transistors Q3 and Q4 is gain-times higher than a difference between the voltages V2 and V3. The signal V4 developed at the collector of the transistor Q3 can be expressed as follows, if the gain of the differential amplifier is A1: ##EQU2##
Therefore, it is understood that the output voltage V4 inputted to the first pair of the input terminals of the multiplier 3 is 90 degrees in advance of the output voltage V2 inputted to the second pair of the input terminals of the multiplier 3. FIG. 2 is a vectorial diagram showing the phase relationship among the output voltages V1 to V4.
In the conventional phase shift circuit using a ceramic resonator for the FM detecting circuit of quadrature detection method, since the number of transistors connected in series is large; that is, since two base-emitter voltages Vbe (of the transistors Q1 and Q3) and one current source (I3) are connected in series between the dc supply voltage Vcc and the ground, a dc supply voltage of at least 1.6 V is required to drive the phase shift circuit. This is because, for instance, the voltages Vbe of the transistors Q1 and Q2 are about 0.7 V and the collector-emitter voltage Vce of a current mirror circuit which constitutes the current source I3 is about 0.2 V.
In the case of portable radio receivers, however, it is desirable to drive the phase shift circuit by a single battery of about Vcc=0.9 V, so that there exists a need of a phase shift circuit activated by a lower voltage.